The principal liquids, the action of which upon solids has been examined, are water, alcohol, ether, petroleum, volatile oils, fixed oils, mercury, alkalies, and the acids' which have been already noticed.

a. Water enters into combination with solid bodies in two states. In the first, the proportion of solid matter exceeds that of water, and the liquid becomes a part of the solid body without rendering it liquid; in the second, the solid is much exceeded by the quantity of fluid, which liquefies it, and imposes its peculiar form upon the compound. The products of the first state are denominated hydrates : the second constitute solutions.

I. Table Of Hydrates, Or Compounds Of Solid Bodies And Water, Still Retaining The Solid Form

1. Sulphur is found native in the state of a hydrate; but the hydrate most generally known is precipitated sulphur. (Part iii.p. 389.)

2. Metallic oxides, when in the state of hydrates, are powders possessed of very intense colour, having usually a strong taste, and being easily acted upon by acid or alkaline solutions.

3. Earthy hydrates are powders, as, for example, quicklime, and in some cases they are crystals.

4. Alkaline hydrates are what are commonly termed the crystals of alkalies.

5. Acid hydrates are those acids which are generally procured in a solid state, and known under the name of crystallized acids.

6. Saline hydrates comprehend the whole class of saline preparations, whether assuming the form of crystals, powders, or solid masses.

7. Hydrates of hydrosulphurets are the crystallized hydro-sulphurets.

8. Soaps are hydrates, water being always present in them as a constituent.

9. Many animal and' vegetable solids.

In the two last classes the proportion of combined water does not appear to be determinate, although this is the case with all the others.

Solution.-During solution, both bodies, or the solid and the liquid, act mutually upon one another at the same time; and the force exerted by each is equal to its mass. The action goes on at the point of contact only: hence, as far as the mass is concerned, the quantity of liquid has no effect in hastening the solution. When a solid body is plunged into a liquid, if the affinity between them be weak, the combination of the two goes on as long only as the force of the affinity is able to overcome the force of cohesion of the particles of the solid; when it stops, the compound remains solid, and is consequently a hydrate. But if the affinity be strong, the cohesion of the solid is gradually destroyed, and its particles, being united with those of the liquid, are dispersed equally through it, forming a solution. By the addition, however, of new portions of the solid, the action of the liquid is gradually weakened; and at length, being unable to overcome the cohesion of the solid, no more of it is dissolved. In this case, the sums of the force of the attraction of affinity exerted between the solid and the liquid, and of the force of the cohesive attraction of the particles of the solid for each other, are balanced; and the liquid is said to be saturated.

The union of the two bodies is accompanied by the usual phenomena of chymical combination. If a portion of the liquid be now abstracted (as, for example, by evaporation), the force of the cohesive attraction of the particles of the solid becomes again superior to the force of the affinity which separates them, so that the solid is reproduced. When this is slowly accomplished, it produces crystallization, the phenomena of which have been already noticed.

In the formation of hydrates the increase of density is often very great, and much caloric is evolved. Thus, hydrate of lime is specifically heavier than pure lime. Hydrate of alum, which is simply crystallized alum, has a specific gravity of 1.7065; but when its water is driven off by calcination, the gravity is reduced to 0.4229; and crystallized nitrate of potash, or hydrate of nitre, is of the specific gravity 1.9639; but nitre deprived of water is only 1.7269.

The density of solutions is greater than the mean, when pure solids are employed; but when the hydrates are dissolved, the specific gravity is more generally less than the mean. The following useful tables drawn up by Hassenfratz, show the specific gravity of saline solutions containing different proportions of salt, at 550.1 By consulting them, we can readily know the exact quantity of salt contained in any saline solution, of a specific gravity corresponding with the numbers marked in the tables; and when the gravity of the solution is not found in the tables, its saline contents can still be found by calculation.

Table Of Saline Solutions

1 The salts were generally in a crystallized state. The column belonging to each salt terminates at the point of saturation at the temperature of 55°.

Table Of Saline Solutions-Continued

Table Of Saline Solutions-Continued

It is necessary to keep in view, that the solvent powers of water are augmented by an increase of temperature, and that the proportions in the foregoing tables are such as take place only at a temperature of 55°.

If a new substance be added to the saturated aqueous solution of another substance, the result is different, according to the nature of the matters employed. Sometimes the second substance is not dissolved: thus, a saturated solution of muriate of lime at 60° cannot dissolve any common salt. Sometimes the whole, or a part of the new solid, is dissolved, without any of the already-dissolved solid being lost or precipitated: thus a saturated solution of nitrate of potassa at 51° can dissolve more chloride of sodium than can be dissolved by pure water, and the same is the case with nitrate of soda; but, in the latter case, a great portion of the nitrate is precipitated. Sometimes the new solid is dissolved at the expense of the whole of the substance already dissolved, which is consequently precipitated : thus, if a sufficient quantity of chloride of sodium be added to a saturated solution of muriate of ammonia at 61°, the former salt is dissolved, but the whole of the latter precipitates during its solution.

The last result, however, does not take place at every degree of temperature; for, at a boiling heat, chloride of sodium is separated by those very salts which it precipitates at a low temperature.

b. Alcohol acts less extensively upon solids than water; and it forms no solid combinations similar to the hydrates.

Table of the pharmaceutical solids which alcohol is capable of dissolving.

1. Sulphur.

2. Phosphorus, and its compounds.

3. Fixed alkalies.

4. Some of the alkaline earths in minute proportions.

5. Most of the solid acids.

6. The vegetable alkaloids.

7. Piperina.

8. Many salts.

9. Alkaline sulphurets.

10. Alkaline soaps.

11. Tannin.

12. Resins.

13. Guaiacum.

A mixture of water and alcohol appears to possess greater energy as a solvent, in many cases, than is possessed by either of them in a separate state.

c. The action of ether upon solids is still more limited than that of alcohol. It dissolves volatile oils, resins, and some vegetable alkalies.

d. The actions of petroleum, volatile oils and fixed oils, have been too little investigated to permit any general deduction.

e. The action of mercury as a liquid is altogether confined to the metals, for many of which it has a considerable affinity, and forms compounds with them, which are denominated amalgams. None of these are objects of pharmacy.